Means for producing an incandescent image



Nov. 7, 1939.

P. T. FARNSWORTH ET AL. MEANS FOR PRODUCING AN INCANDESCAENT ILAGE mew m WWN m la@ @u an Mz f El MBMWPM Original Filed May 6, 1956 o method, within the scope of the appended claims. Considered from the aspec Patented Nov. 7, 1938 UNITED. STATES PATENT OFFICE MEANS Fonrnonuim mcANDEscEu'r y I Philo T. Farnsworth, Springiield Townshi Montgomery County, and Bernard C. Gardner, Philadelphia, Pa., assignors, by mesne assignments, to Farnsworth Television 6r Radio Corporation, Dover, Del., a corporation of Dela-4 ware Original application May 6, 1936, Serial No. 78,188. Divided and this. application September 20, 1937,

' Serial No. 164,624

3 Claims. (ol. 25o- 164) This invention relates to a method of pro- Figs. 3 and 4 are sectional views illustrating ducing an incandescent image for use as a primodified forms of the screen of our invention. mary light source in the projection of images, Fig. 5 is a diagram showing our invention used such as oscillographic traces, television images, as a light source for picture projection.

5 or the like. For many years it has been customary to form 5 This application is a division of our prior the luminescent screen used in cathode ray oscilapplication entitled Luminescent screen, Serial lographs and the like, of materials which fluoresce No. 78,188, led May 6, 1936, since matured into under the bombardment of the rays, 'such as U. S. `Patent No. 2,098,000, issued November 2, calcium tungstate or barium platinum cyanide,

o 1937, which is a substitute for original applicawith or without an admixture of a phosphorescent 1 tion also entitled Luminescent screen, Serial material Such as Zinc. sulphide. Luminescent No 614,501,111@ May 31, 1932, The primappliscreens of this character have the advantage of cation referred to disclosed and claimed an ap- Very great luminous sfciencies ,With W input paratus, whereas the present application is di` p ower. Where the electron stream carries rela- V rected solely to a method of operation whereby an tlvely 10W current and travels with moderate 15 incandescent image can be produced Within a velocity, its trace on the screen is clearly visible cathode ray tube. under conditions of moderate external illumina- Among the objects of our invention are; To tion. If,however, the attempt is made to increase produce an incandescent image; to produce 'ahe.luunllfl2.111911l beyond a Certain Very moderate 2 an image having suiicient brilliance that it may nl mt only sllglflt increases are Obtained even 20 be used as a primary light source 'for projection; Wlth Very great Increases 1n the Input DOWGI, and to provideamethod of producing an incandescent there is a Strong tendency fOr the material of image suitable for projection; and to provide a the screen to break down and become inactive single and emcent method of'creang an imam under the bombardment.l Moreover, the light -ldescent image in a cathode ray tube, which can from the Scfeen 1S usually Very definitely colored, 25 be utilized as a primary light source in the focus Suaufhavmg a blulsh 01' g1`e 11sh tmgs WhlChv of a projection lens without substantial image make? 1t undexrable fg!" tielevlsmn use' Where a distortion, keystonng, or angular relationships. dg; st'hetic ege 1S annffd at- The Pumps@ Our invention possesses numerous other objects gh vent on 1s to provde a Screen- Whlch and features of l advantage, some of which, toqws ine trace of tpe Cathode ray beam m pfre gether with the foregoing, will be set forth in the Whlte light and .'hwh 1S capable of almost n' following description of sp ic apparatus erngemme mcreasgs m 1th? amount gf power Whmh bodymg and utmzingbur novel method It is 1t transforms intol visible radiation. Although the method here employed is less sensitive on therfore to be understood that 05. method is small power inputs than is the fluorescent screen, 35

aiilhfaple t0 0the1`- apparatus and that We do we have found that with large power inputs the not llmlt 0uTSe1VeS, m mY Way to the apparat@ eiciency may approach that of the conventional of the present applicatlon, as we may adopt vari- Screen, giving an output power vastly greater ous other apparatusempodiments utilizing the than is possible with conventional methods.

iii)

. t of the apparatus 40 fis examples pf other apparatus Whlch can pe employed, our invention comprises the combinautilized to practice our method, reference may be tion with a source of Cathode rays of a screen had. fOr example, ,t0 Umted States Paten? N0- having a surface of refractory material which is 2,066,070, Issued December 29, 1936, t0 PhllO T- capable of being raised to incandescence by the Farnsworth and Harry S- Bamford 0n an n' impact of the rays, this material being so dis- 45 candesccnt hght SOuICe, 01 t0 the Copendmg posed as to prevent material thermal conduction FarnsWOrth application entitled Lum-inescnt to adjacent portions of the surface, i. e., so disscreenand method of use, Serial NO- 055,734, posed that the heat loss 'from an incandescent led February 3, 1933- surface element by radiation is greater than its Referring to the drawing: loss by conduction to adjacent surface elements. 50 l Flg- 1 is a lngitudinal sectional View thl'Ough These requirements of the. screen are met in suf# an oscillight or television receiver tube embodycient degree to be operative by a sufliciently ing 'this invention. thin sheet of refractory metal of low thermal Fig. 2 is asectional view on a larger scale conductivity, such as a sheet of molybdenum foil, through the luminescent screen shown in Fig. 1. but the' luminous eflciency in this case is low. 55

a separate side seal I4,

The preferred type of screen is formed by impregnating a pile fabric, such as a nely woven short pile rayon velvet, with salts of a metal or metals whose oxides are poor emitters of infrared or heat rays, burning out the fabric, and converting the salts to the oxides. Salts of calcium, magnesium. thorium, cerium, zirconium and uranium are examples of the class of materials used to impregnate the fabric, the preferred combination being a mixture of thorium with a small amount of uranium. Screens of this type, bombarded from the pile side and preferably viewed from the same drawing comprises the usual evacuated envelope supplied to the anode. beam is regulated by a control electrode or grid I5, mounted on a lead I6 sealed through the stem.`

The longitudinal support rods I carry, at their upper or distal ends, a ring I1 which supports the luminescent screen I9. The ring I1 is of U-shaped cross section, the arms of the U being crimped to hold the fabric upon lwhich the screen is formed. This is preferably a cut rayon velvet of very fine weave and short pile, the general rule being the finer the weave the more satisfactorythe screen. The fabric is mounted with the pile 2Iitoward the' electron gun. It is impregnated with a mixture of salts whose oxides form the light emitters or,radiators of the compositive potential to be pleted screen. I'hese materials are preferably aI mixture whose basic radicals comprise approximately 99 per cent thorium and 1 per cent uranium. To this may be added a small proportion of aluminum and/or beryllium salt. In general, the

are converted into the oxide.

The fabric is thoroughly impregnated with the salt solution, is thoroughly dried, and is then oxidized or "burnt oi in the same manner as is the ordinary incandescent gas mantle. In this process practically all oi' the original material of the fabric is removed, leaving a structure of oxand then oxidizing the screen,

particles which retains the genoriginal fabric matrix. A large in the process.

ide crystals or eral form of the shrinkage occurs uranium oxide.

The various other radiators mentioned, such as lime or calcium oxide, zirconium will operate, but at greatly less eiliciency, as comthe sake of clarity. When an alternating current of a different freis manifested in heating of the screen.

Since the individual fibers of the pile to its small cross section transverse to the direction o1' the heat ow.

Since radiation is practically the only channel of heat to darkness.

Using the finest Weave velvets as substituted for the 15 the a second, the excitation occurring twenty times per second. Screens based on coarser fabric are correspondingly slower, but in any case the heating period is extremely short in comparison with i the radiating period, so that the heating period may be considered as instantaneo 25, as shown in Fig. 3, be pile fabric, the screen becomes too slow for satisfactory television use,

If a plain fabric `0 although it is still sufliciently rapid for even the most rapid ordinary oscillographic work. Where,

as in Fig. 4, a metal foil 26, mounted upon a ring l1', and having its inner space coated with parprinciples here set forth to a greater or less degree.

It will be seen that the success or failure of a screen of the type here described depends on the proper coordination of heat generation and heat dissipation from the incandescent elements of the surface. The effect of a cathode ray beam in heating the anode has long been known, and modern X-ray tubes `are customarily operated with their anodes at incandescence. In this case, however, the beam falls constantly on one spot, and the heat generated diffuses quickly throughout the massive target.

In the screen of our invention, however, the

beam is constantly moved from one elementary 1 path of the beam at any instant is so small that its rise to incandescence is pra tically instantaneous. Furthermore, conduction to adjacent elements is so restricted that heat loss by radiation cools the bombarded area before'the incandescence can spread appreciably. Were the heat supplied continuously to one spot there would, of course, be spreading of the luminous area no matter how great the thermal resistance. The practically continuous movement of the trace is,

therefore, an essential feature in the operation of a screen of this character.

With screens of the pile type, the illumination produced is of the order of 2 candle power per watt of input energy. With 70 watts input the u television picture produced on a screen of this type an inch and a half to two inches square, is

so brilliant that the screen cannot be viewed directly. It may, however, be projected by a suitable lens system '29, as shown in Fig. 5, upon a projection screen 30 or nat surface having a high coefcient of reflection. Under these circumstances the eect of the illumination pro- `said bombarded surface to produce descent image larger than the cross section of duced is substantially twice to four times that available with home moving-pictureV outfits using' a 100 watt lamp. The actual luminous iiux produced is, of course, muchxless` than that of the home projection outfit, but in the case of the screen of our invention it is the image of the luminous source itself which is projected, whereas in the case of the moving-picture nlm, approximately 90 per cent of the infalling `light is absorbed by the film. It is therefore possible, using screens of this type, to project television images in rooms having a fair degree of general illumination, and to a size of from one to five feet square, depending upon the general illumination level and the amount of the power expended in the cathode ray beam.

We claim:

l. Means for producing an incandescent image for projection by a lens system, comprising an envelope containing a thin opaque and planar screen having one surface positioned in the focal plane of said lens system, means for bombarding an opposite surface separated from said rst surface by the thinnest section of said screen with a defined cathode ray beam of sufcient intensity to raise both the surface bombarded and the surface presented to said lens to incandescence, and means for moving said beam over an incansaid beam. Y

2. In an image projecting system having an envelope containing a primary light source coaxially positioned between an anode and cathode cooperating to produce abeam of cathode rays,

and a lens system, said beam being moved in two dimensions over an image field, said primary light source comprising an opaque screen of incandescible material throughout its thickness and of such thickness that it can be raised to incandescence throughout its thickness and on both its opposite faces by impingement therewith of the beam of rays from said gun, whereby an incandescent image is formed on both sides of said screen.

3. In an image projecting system wherein an optical image is produced within a cathode ray tube and projected onto a viewing screen by an outside lens system, image producing means comprising an envelope containing an anode and cathode cooperating to'produce a cathode ray beam of high intensity, a thin opaque screen of incandescible material in the path of said beam and positioned at right angles to the normal undeected path of said beam, at right angles tov I PHILO T. FARNSWORTH. BERNARD C. GARDNER. 

